ipv6: on reassembly, record frag_max_size

IP6CB and IPCB have a frag_max_size field. In IPv6 this field is
filled in when packets are reassembled by the connection tracking
code. Also fill in when reassembling in the input path, to expose
it through cmsg IPV6_RECVFRAGSIZE in all cases.
Signed-off-by: NWillem de Bruijn <willemb@google.com>
Acked-by: NEric Dumazet <edumazet@google.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

net/ipv6/reassembly.c

@@ -211,7 +211,7 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 {
 	struct sk_buff *prev, *next;
 	struct net_device *dev;
-	int offset, end;
+	int offset, end, fragsize;
 	struct net *net = dev_net(skb_dst(skb)->dev);
 	u8 ecn;
 
@@ -336,6 +336,10 @@ static int ip6_frag_queue(struct frag_queue *fq, struct sk_buff *skb,
 	fq->ecn |= ecn;
 	add_frag_mem_limit(fq->q.net, skb->truesize);
 
+	fragsize = -skb_network_offset(skb) + skb->len;
+	if (fragsize > fq->q.max_size)
+		fq->q.max_size = fragsize;
+
 	/* The first fragment.
 	 * nhoffset is obtained from the first fragment, of course.
 	 */
@@ -495,6 +499,7 @@ static int ip6_frag_reasm(struct frag_queue *fq, struct sk_buff *prev,
 	ipv6_change_dsfield(ipv6_hdr(head), 0xff, ecn);
 	IP6CB(head)->nhoff = nhoff;
 	IP6CB(head)->flags |= IP6SKB_FRAGMENTED;
+	IP6CB(head)->frag_max_size = fq->q.max_size;
 
 	/* Yes, and fold redundant checksum back. 8) */
 	skb_postpush_rcsum(head, skb_network_header(head),